Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Tachyarrhythmias generally include supraventricular tachycardia (SVT, including atrial tachycardia, AT) and ventricular tachycardia (VT). Fibrillation is a form of tachycardia further characterized by an irregular heart rhythm. In a normal heart, the sinoatrial node, the heart's predominant natural pacemaker, generates electrical impulses that propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles contract in the normal atrio-ventricular sequence and synchrony to result in efficient blood-pumping functions indicated by a normal hemodynamic performance. VT occurs when the electrical impulses re-enter the atria from the ventricles to form a self-sustaining conductive loop or when a natural pacemaker in a ventricle usurps control of the heart rate from the sinoatrial node. When the heart rate reaches certain levels, the ventricles contract before they are properly filed with blood, resulting in diminished blood flow throughout the body. This condition becomes life-threatening when the brain is deprived of sufficient oxygen supply. Ventricular fibrillation (VF), in particular, stops blood flow within seconds and, if not timely and effectively treated, causes immediate death. In very few instances a heart recovers from VF without treatment.
Implantable cardioverter defibrillators (ICDs) are used to treat most tachyarrhythmias, including AT, VT, and VF. An ICD is an implantable medical device that delivers an electric shock pulse to terminate a detected tachyarrhythmia episode. The electric shock pulse depolarizes portions of the myocardium and renders it refractory. The energy of the shock pulse is provided by one or more defibrillation capacitors of the ICD. Following a detection of an episode of tachyarrhythmia, the one or more defibrillation capacitors are charged to a programmed capacitor charging level in preparation of the possible delivery of a shock pulse. The maximum defibrillation energy level of the LCD, referred to as the ICD energy level, is the maximum level up to which the capacitor charging level can be programmed. Thus, the ICD energy level determines the maximum amount of energy deliverable with each shock pulse.
During the implantation of an ICD to a patient anticipating tachyarrhythmia episodes, a defibrillation threshold (DFT) test is performed to determine the DFT, which is the energy level of the shock pulse required to terminate a tachyarrhythmia episode of that patient. The energy level of each shock pulse is then programmed to a level exceeding the DFT by a safety margin. The programmable energy level for each shock pulse is limited to the ICD energy level. Thus, an ICD with an ICD energy level higher than the patient's DFT by the safety margin is to be chosen for the patient.
For an ICD, a higher energy level means bigger size, longer capacitor charging time (and hence longer delay in responding to a tachyarrhythmia detection), and shorter device longevity. Because the DFT varies significantly from patient to patient, there is a need for ICDs that are energy-efficient over a range of substantially different DFTs.